Trees of the Last Ice Age

According to Gerhart et al. (2012), "at the peak of the last glacial period, atmospheric CO2 concentrations ranged between 180 and 200 ppm, which are among the lowest concentrations that occurred during the evolution of land plants (Berner, 2006)," and they say that "when grown at glacial vs. modern CO2, modern C3 plants show 40-70% reductions in photosynthesis and biomass production (Polley et al., 1993; Sage and Coleman, 2001), 20-30% lower survival (Ward and Kelly, 2004), and may even fail to reproduce (Dippery et al., 1995)."

Against this backdrop, physiological responses to changes in atmospheric CO2 concentration that are manifest in stable carbon isotope ratios of alpha-cellulose extracted from individual tree rings were determined for glacial-age Juniperus (juniper) trees preserved in the Rancho La Brea tar pits of Los Angeles, California (USA), which were 14C dated to 14.5-47.6 kyr BP (thousand years before present), after which these responses - which ultimately led to determinations of intercellular/atmospheric CO2 ratios (ci/ca) - were then compared to those of modern-age Juniperus trees growing in the Angeles National Forest and higher elevation sites in the San Bernardino National Forest, both of which sites are located close to the La Brea tar pits.

At the completion of the analysis, Gerhart et al. report that Juniperus trees "showed constant mean ci/ca between the last glacial period and modern times, spanning 50,000 years," and on the basis of modernca values obtained from direct atmospheric measurements (Keeling et al., 2009) and the Taylor Law Dome ice core (Etheridge et al., 1996) and glacialca values derived from the Vostock and EPICA Dome C ice cores, they discovered that glacial Juniperus trees "exhibited low ci that rarely occurs in modern trees," i.e. 106 ppm vs. 168 ppm.

In describing the significance of their findings, the five researchers say that their study "provides some of the first direct evidence that glacial plants remained near their lower carbon limit until the beginning of the glacial-interglacial transition," which lower limit appeared to them to be 90 ppm CO2, below which value it appeared that "juniper trees may not maintain a positive carbon budget for basic physiological functions for survival," as is also suggested by the corroborative work of Campbell et al. (2005). Atmospheric CO2 concentrations of today, therefore, are a great benefit to juniper trees, and all vegetation in general.